To unlock this Trophy / Achievement, you’ll need to upgrade the Power of Jotunheim ability by purchasing the Jotunheim Assassin upgrade. You can then use this to teleport assassinate enemies and then quickly teleport away without being seen. You’ll need to do this near a Jotnar World Knot portal, which you can use to fly away with after assassinating two targets. One of the best places to do this is at the Jotnar settlement of Tjarnar Mylna in northern Svaladal, as shown in the screenshot below.

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The location of Tjarnar Mylna in northern Svaladal, where you’ll find plenty of Jotnar.